1. Field of the Invention
The subject invention relates to needles with collapsible sheaths and tip guards to prevent accidental needle sticks.
2. Description of the Prior Art
A needle cannula includes opposed proximal and distal ends and a lumen extending therebetween. Normally, the proximal end of the prior art needle cannular is mounted to a hub for placing the lumen in communication with a medical instrument, such as a hypodermic syringe, while the distal end of the prior art needle cannula is typically beveled to define a sharply pointed tip. Health care workers and patients can be stuck accidentally with the distal tip of a needle cannula. An accidental stick occurring prior to use of the needle cannula is painful and can cause infection. An accidental stick occurring after use of a needle cannula can further transmit disease.
Most prior art needle cannulas are provided with shields that are intended to reduce the risk of accidental sticks. Some prior art shields include a resiliently collapsible sheath that surrounds the needle cannula prior to use. In use, these sheaths extend distally beyond the needle cannula, and consequently will contact the skin of the patient prior to making an injection. The sheaths expand radially and collapse axially as the needle cannula is moved toward and into the patient.degree. The sheath then resiliently returns to an axially extended, radially contracted condition as the needle cannula is withdrawn from the patient. An example of a prior art collapsible sheath for a needle cannula is shown in U.S. Pat. No. 4,139,009. Prior art needle cannulas with collapsible sheaths present a false sense of security. Health care workers may assume that the covered needle is safely protected. However, any proximally directed force on the distal end of the sheath will expose the needle cannula and create the potential for accidental needle sticks.
More recent prior art includes a collapsible sheath that is closely surrounded by a coil spring. The proximal end of the coil spring is fixed near the proximal end of the sheath. However, remaining portions of the coil spring can be collapsed in a proximal direction independent of the sheath. The sheath functions substantially as in the above described U.S. Pat. No. 4,139,009 when the coil spring is collapsed. However, the coil spring can be released to expand axially and closely surround the sheath. The coil spring is intended to prevent the radial expansion of the sheath that is required to axially collapse the sheath. Hence, the coil spring is intended to keep the sheath in its extended position. This combination of a sheath closely surrounded by a coil spring is shown in U.S. Pat. No. 4,998,922.
The above described combination of a collapsible sheath closely surrounded by an independently collapsible coil spring is still not a completely satisfactory approach to the aforementioned concerns. For example, the sheath may not completely return to its fully extended position. The coil spring, therefore, will not have a free path of travel into its fully extended condition. Ideally, the expanding coil spring will generate the full extension of the sheath. However, the close engagement between the sheath and the coil spring can cause the coil spring to bind prior to reaching its fully extended condition, and therefore will leave the distal tip of the needle exposed. Even when the sheath and the coil spring are fully extended and cover the distal tip of the needle, proximally directed forces on the sheath can cause sufficient deformation of the sheath, the coil spring and/or the needle cannula to re-expose the distal tip of the used needle cannula. Additionally, the requirement of disposing the coil spring axially around the sheath may be considered aesthetically or functionally undesirable for some applications. Finally, the known combination of a collapsible sheath closely surrounded by a coil spring provides no convenient way to intentionally re-expose the needle cannula. For instance, an intentional re-exposure is desirable in situations where a drug, such as an anesthetic, is administered in small doses over a period of time in accordance with the needs of the patient. Any attempt to intentionally re-expose the needle cannula shown in U.S. Pat. No. 4,998,922 could generate the inadvertent needle stick that the sheath and coil spring are intended to avoid.
Accordingly, there is a need for a device which provides safe, reliable re-exposure of a needle cannula during use while alleviating the risk of accidental needlesticks associated with conventional needle shields, and reducing the problems associated with prior attempts.